1. Field of the Invention
The present invention relates to programming metal-to-metal antifuse structures. More particularly, the present invention relates to methods for programming amorphous carbon metal-to-metal antifuse structures.
2. The Background Art
Antifuses are devices which generally comprise a pair of conductive electrodes sandwiched about an insulating antifuse material layer. Prior to programming, an antifuse element is an open circuit that exhibits a resistance of a few gigaohms. A programming process disrupts the antifuse material and forms a filament between the two electrodes which creates a low-impedance connection of a few ohms to a few thousand ohms between the two electrodes. Programming voltages range from a few volts to about 20 volts.
Metal-to-metal antifuses are well known in the art. Under specific conditions where excessive current is placed across a programmed antifuse, it is known that metal-to-metal antifuses will exhibit a phenomenon wherein the antifuse will revert to its off state through the destruction of the conductive filament. This failure mode often manifests itself after a period of time during which the antifuse links appear to have been properly formed. This failure mode is commonly known as xe2x80x9cread disturbxe2x80x9d because the excessive parasitic current occurs during the read state of the antifuse.
One factor which contributes to read disturb is the presence of any significant quantity of aluminum in the antifuse conductive links due to electromigration of the aluminum. Metal Barrier layers which serve to block aluminum flow into the antifuse material layer of various materials and various thicknesses have been proposed. The barrier materials, between the aluminum and the amorphous silicon, provide essentially all of the conductive material forming the conductive filament in the programmed antifuse. An antifuse formed with such a barrier metal link material can tolerate more current and have a higher reliability than an antifuse formed without the barrier metal link material.
To minimize read disturb, methods of antifuse programming have also been devised. In one proposed method, the direction of the antifuse programming is implemented such that a series of asymmetrical programming pulses having an amplitude in one direction greater than the amplitude in the other direction is applied so that the antifuse is more likely to program during the greater pulse. If it fails to program on the first greater pulse, it probably won""t program on the first lesser pulse and will likely program on a subsequent greater pulse. In this way it can be assured that the antifuse will only disrupt while the current is flowing in one particular and predetermined direction.
In another method, the programming current is increased to a level which is much greater than the operating current. With high programming current, more power is generated during formation of the conductive filament, resulting in a filament having a larger diameter. This provides immunity from major material transport inside the conductive filament, because for the same operating current, the current density will be lower. This is essentially the approach currently taken in programming amorphous silicon metal-to-metal antifuses.
It is generally accepted that amorphous silicon metal-to-metal antifuses have a known switching phenomenon corresponding to a ratio of read current to programming current of about 0.8 (switching ratio) which requires strict adherence to two design rules. First, for nodes with enough programmed antifuses to be considered heavily loaded, the subsequent high operating current requires that a high programming current be employed. As a result, larger transistors which add to the die size, are required to provide the higher programming current. Second, amorphous silicon metal-to-metal antifuses have a minimum required programming current of greater than 5 mA to maintain a stable resistance. Below 5 mA, the programmed antifuse resistance is non-linear, and creates a diode-like effect.
In an embodiment of the present invention, a first low programming current pulse is applied to an antifuse in a first direction for a predetermined amount of time followed by a second low programming current pulse applied to the antifuse in the opposite direction. A high soaking current pulse is then applied to the antifuse in the first direction for a predetermined amount of time followed by a second low programming current pulse applied to the antifuse in the opposite direction for a predetermined amount of time. The method generally of low current followed by a higher current provides a relatively lower and tighter distribution of antifuse resistance than a high programming current followed by a high soaking current.